The present invention relates to a digital switch serving a number of terminal units for switching signal traffic therebetween, said traffic being logically divided into data time slots and control time slots with control packages or packets, comprising
a switch memory for receiving said data time slots, PA1 a control memory containing information for facilitating through connection of said data time slots in said switch memory, PA1 a control memory terminal for receiving said control time slots and control packages located therein, and writing information into the control memory, and for transmitting acknowledgement packages to called and calling ones of said terminal units, PA1 an occupied/unoccupied memory cooperating with said control memory terminal and storing actual status of each of said terminal units. PA1 a queue system associated with said control memory terminal and said occupied/unoccupied memory, said queue system including memory means and control logic means for effecting, PA1 in case a first terminal unit tries to establish connection to a second terminal unit, which is occupied, PA1 queueing of a call request and transmission to said first terminal unit of an acknowledgment "request for connection queued" and, at the receipt of a request for disconnection from said second terminal unit, PA1 direct connection of said first terminal unit to said second terminal unit, and PA1 transmission of acknowledgements to said first and second terminal units with the information "connection effected" and "request effected" respectively
For market considerations telephone systems have been costwise orientated towards normal telephony. Today it can be seen how new requirements on data communication services grow increasingly stronger. On the one hand this depends upon the telephone network having become "the biggest distributed computer system of the world" with requirements for integrated signalling system (number 7), and on the other hand upon new services such as e.g. ISDN (Integrated Services Digital Network) now being close to their realisation. To this comes X.25 networks and similar.
In order to costwise meet the expected expansion of the data communication services mentioned above in an optimum way and to enable "integrated control routes" in a telephone exchange for communication between internal control processors, it is desirable in the next generation of telecommunication systems to build switches with "Fast-Circuit" properties. By this is meant a switch, where the nodes connected to the switch, independently of whether it is a simple subscriber line card or a powerful processor, with lightning rapidity shall be able to transmit a request for connection to the switch, which performs the required operation and transmits an acknowledgment back. Then a route has been established through the switch. An example of a switch with "Fast-circuit" properties is described in PCT/SE92/00819. Due to its properties this switch can also serve as a data communication switch with high performance.
One of the most difficult problems in data communication switching is to achieve a fair distribution when several transmitters want to transmit to a receiver simultaneously in e.g. a star network or correspondingly for buses/rings when all nodes share the same media and want to transmit simultaneously. As examples of solutions for buses, contention detection with "back-off" algorithms for e.g. ETHERNET-networks and "Tokens" can be mentioned, which can be reserved.
In star networks the problem is usually solved by buffering the whole message in the star point, which then can be subject to a temporary congestion, with overspill as a consequence. Buffering furthermore works badly in other networks than those having moderate to low transmission rates (around 64 kb/s).
For star connected switches, as the one in the PCT patent application mentioned above, the problem also exists, but there it is for cost reasons not possible to buffer the whole data message (cost, volume, power and data rates).
This results in that two nodes can outcompete a third node in trying to transmit to the same receiver.